The invention relates to a circuit, which can be completely integrated in an electronic module (chip), for data signal recovery and clock signal regeneration from an incoming serial data signal stream with a retiming circuit. The circuit uses a PLL (phase locked loop) regulating stage which is provided with a voltage-controlled oscillator (VCO) and to which the serial data signal stream is fed.
The invention is directed, in particular, at the recovery and the retiming of data signals and clock signals, respectively, from serial data streams, e.g. in transceiver circuits for ATM (asynchronous transfer mode), SONET (synchronous optical network) and SDH (synchronous digital hierarchy) applications.
It is known to realize the reconditioning of data signals and clock signals with the aid of a PLL regulating stage and retiming flip-flop. There are diverse types of phase and frequency detectors for this purpose. Reference is had, in this context, to a paper by Hans-Jxc3xcrgen Herzog entitled xe2x80x9cAuswahl von Bausteinen fxc3xcr die Daten- und Taktregenerierung in Telekom- und Datennetzenxe2x80x9d [xe2x80x9cSelection of modules for data and clock regeneration in telecommunications and data networksxe2x80x9d], published in the journal xe2x80x9cHF-Praxisxe2x80x9d, issue 5, 1998, volume 4, pp. 12-14.
The incoming data signal is generally a serial bit sequence encumbered with noise and jitter. Various requirements are imposed on a transceiver which receives and evaluates this data signal stream, in order that a signal of required quality is produced again on the output side. Two important requirements, which, however, are partly at odds with one another, are the values for the jitter tolerance and for the jitter transfer. The jitter tolerance defines the maximum permissible input jitter which the circuit can still process in a manner free from errors. This value should be as large as possible. The jitter transfer defines the maximum permissible jitter which is allowed to be transferred from the input to the output. It should be as small as possible.
In order to fulfill these jitter requirements, the bandwidth of the PLL regulating loop used in the reconditioning of data signals and clock signals must be adapted to the requirements. A large PLL regulating loop bandwidth is necessary for a large jitter tolerance.
A large bandwidth enables the PLL regulating loop to effect rapid following in terms of the frequency and phase of the incoming signal and thus reliable sampling in the temporal center of a data bit. This fact then also results in the circuit having high input sensitivity.
A small PLL regulating loop bandwidth is necessary for a low jitter transfer. This ensures that the PLL regulating stage does not follow the high-frequency jitter, noise and other interference and thus impair the quality of the recovered data signal.
In order to simultaneously meet both conditions to some extent, one is thus forced to adopt a compromise. In this case, the bandwidth of such a PLL regulating loop is in a very narrow range. Since a PLL regulating stage can in part comprise highly nonlinear components, particularly in the case of completely integrated PLL regulating loops, it is difficult to calculate or realize the bandwidth.
The object of the invention is to provided a circuit, which can be completely integrated on an electronic chip and thus implemented without external circuitry, for the recovery and for the retiming of data signals and clock signals, respectively, from serial data streams, in particular for a simpler construction of ATM, SONET and SDH-conforming transceiver circuits for possible use in signal transmission links in the gigabit range, the jitter requirements being complied with and, consequently, a data signal of required quality, that is to say having a prescribed low bit error rate, being produced again on the output side.
With this and other objects in view there is provided, in accordance with the invention, a circuit for data signal recovery and clock signal regeneration from an incoming serial data signal stream comprising data bits. The circuit, which is completely integrateable into an electronic component, comprises:
a first PLL regulating stage having a voltage-controlled oscillator, an input receiving a serial data signal stream, and an output outputting a clock signal;
a second PLL regulating stage connected to the output of the first PLL regulating stage and in series therewith, the second PLL regulating stage having an input receiving the clock signal and an output outputting an output clock signal;
the first and second PLL regulating stages being independent of one another and each being optimally adjustable separately, whereby the first PLL regulating stage is set at a first bandwidth, and the second PLL regulating stage is set at a second bandwidth smaller than the first bandwidth.
In other words, the objects of the invention are achieved, according to the invention, which relates to a circuit of the type mentioned in the introduction, by virtue of the fact that there is connected downstream of the PLL regulating stage in series a second PLL regulating stage, that the two PLL regulating stages are independent and are each optimally adjustable separately, that the first PLL regulating stage is set in such a way that it has a large bandwidth and is optimized for maximum jitter tolerance, and that the second PLL regulating stage is set in such a way that it has a small bandwidth and is optimized for minimum jitter transfer.
The invention thus solves the problem by connecting two independent PLL regulating stages in series, for each of which the optimum setting is performed separately. The first PLL regulating stage has a large bandwidth and regenerates the level of the incoming signal.
As a result, the signal/noise ratio becomes less critical and the second PLL regulating stage can guarantee error-free data regeneration, even without sampling in the absolute center of a data bit. The second PLL regulating stage has a small bandwidth and can thus be optimized for low jitter transfer.
Complete integration on a single chip is possible since the circuit according to the invention can tolerate relatively large parameter fluctuations of the circuit.
In accordance with an added feature of the invention, the second PLL regulating stage is configured to synchronize the clock signal and the output clock signal and wherein a transition from the first PLL regulating stage to the second PLL regulating stage is performed via a synchronization of the clock signal and the output clock signal realized in the second PLL regulating stage. That is, the transition from the first PLL regulating stage to the second PLL regulating stage is performed by means of synchronization of the two clock signals which is carried out in the second PLL regulating stage. The second PLL regulating stage can be realized in a simple manner and without a high technical outlay on circuitry.
In accordance with an additional feature of the invention, a constant frequency crystal oscillator is provided to stabilize a reference frequency of the first PLL regulating stage.
In accordance with another feature of the invention, the circuit is particularly suitable in a transceiver circuit at an end of a transmission link of a telecommunications and data transmission network.
In the preferred embodiment of the invention, the signal transmission link operates in a gigabit range.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit for data signal recovery and clock signal regeneration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.